I have TP-LINK TD-8901G router and it has 4 LAN ports, but there is no separated WAN port. Also, I have one CISCO 2100 cable modem. So, how I can setup tp-link to use signal from cisco modem and to share Internet link through other LAN ports and wi-fi?

Here is the TP LINK user guide for this model, but i don't know what to look for that.